Steam generator



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aul Pro M P. PROFOS STEAM GENERATOR July 9, 1963 3 Sheets-Sheet 2 Filed June 9. 1959 a Ru/ Pr July 9, 1963 P. PROFOS 3,096,743

STEAM GENERATOR Filed June 9. 1959 3 Sheets-Sheet 3 Fly. .5

United States Patent 3,096,743 STEAM GENERATOR Winterthur, Switzerland, assignor to Sulzer Paul Profos,

Winterthur, Switzerland, a

This invention relates to a steam generator including a least two tube systems each traversed by a part of the working substance and each including a reheater for partialiy expanded steam. Hereinafter reference to water or steam is to be understood as including any other working substance suitable for use in a boiler.

Various arrangements have been proposed for holding at a given level the temperature of reheated superheated steam. For example, it has been proposed to provide movable burners in order to permit change in the location of maximum heating in the evaporator tube system. For control of the temperature to which partly expanded steam is reheated there have also been proposed heat exchangers disposed in the flow path of the partly expanded steam, in which the partially expanded steam is heated or cooled by means of steam emerging from the high pressure end of the boiler. For control of such an exchanger there may be provided an injector arrangement for adding to the high pressure steam a quantity of water before its introduction into the heat exchanger. Alternatively, there may be provided a bypass line around the heat exchanger with a suitable flow control means therein, the bypass line being either in the channel of the steam to be reheated or in that of the high pressure steam which is employed to heat or to cool the steam to be reheated. The temperature to which the partly expanded steam is reheated, hereinafter called the reheat tempera ture, can also be controlled with the help of stack gases, either by means of partial and controlled bypassing of the steam to be reheated around reheater surfaces heated by such gases or by control of the application of such gases themselves to the reheater.

It has also been proposed, especially in steam generators of large capacity, to divide the heating surfaces into separate parallel tube systems with separate control of heat application thereto. This has been done in view of the difliculty of distributing heat transfer uniformly to the various parallel channels of the steam tube system. However it is in precisely these large generators that the range of control available, with respect to the reheat temperature, with the various prior art means above described has proved insufficient to hold the reheat temperature at the desired levels over the wide range of loads encountered.

The invention surmounts this difficulty, without substantial increase in the requirements of the system for control apparatus. In a steam generator according to the invention there are provided, for control of the superheated temperature to which the steam is reheated, means which are capable of altering in the same sense or direction the heat transfer in all such tube systems, i.e. of raising the heat transfer in all, or lowering it in all.

Such means will hereinafter be referred to as coupled control means, in contra-distinction to individual or differential control means operating on one or more tube systems to the exclusion of others, or raising the heat transfer in one while lowering it in another.

The invention further provides, in order to extend the range of reheat temperature control, supplementary coupled control means operating on each of a plurality of tube systems.

All of these control means are subjected to the effect of control impulses dependent on the temperature of reheat achieved in at least one of the tube channels. Lastly, for individual control of the temperature of reheat in one of the tube channels, a further control signal representative of that temperature is in that channel superposed on the control signal which is applied to the additional means above referred to.

The invention is based on a recognition of the fact that the range of load values over which the reheat temperatures may be controlled can be substantially increased by the provision of additional or supplementary means to change in the same sense the heat transfer rate in all of the tube systems and on the further fact that provision of separate means for individual control of the reheat temperatures in a single one of the tube systems requires only a minor increase in the total amount of control apparatus.

A-dvantageously there are employed, as a first means for control or adjustment in the same sense of the rate of heat transfer in all tube systems, a plurality of movable burners coupled for motion together in a common firing space to which all of those tube systems are exposed.

Advantageously moreover there may be provided heat exchangers in the flow path of the partially expanded steam, which exchangers are heated or cooled with high pressure steam. With such an anrangement, a means provided for coupled control of heat transfer in the plural tube systems of the boiler may take the form of coupled Water injectors for the addition of water to the high pressure steam before its introduction into 'the heat exchangers. Alternatively the coupled control means may take the form of rate of flow control devices disposed in bypass connections for the high pressure steam around the heat exchangers.

In another embodiment of the invention one of the coupled control means may take the form of rate of flow control devices coupled together, and disposed in bypass lines for the steam to be reheated, these lines being connected around at least a portion of the reheater heat transfer surfaces. In a steam power system according to the invention including for each of plural tube systems one reheater having heat exchange surfaces in contact with the products of combustion, one of the coupled control means may take the form of means for control of the flow of stack gases which divert a part of the stack gases around at least part of the reheater surfaces.

In another embodiment of the invention employing recirculation of stack gases a coupled means for control of reheat temperature in plural tube systems may take the form of means to control the quantity of stack gases so recirculated.

Advantageously, in accordance with the invention, provision is moreover made so that the supplementary means for coupled control of the reheaters come into operation to change the heat transfer in the reheaters only after the first means for such control have reached the limit of their range of adjustment.

Operation of both the first and supplementary coupled control means can be made to depend upon control signals derived from an average of the reheat temperatures at all reheaters. According to another alternative a single control impulse may be employed, derived from the reheat temperature of the reheater in a single one of the tube systems.

Further according to the invention there may be provided, in a steam power system having plural tube systems, a first coupled control means for control of reheater heat transfer in the form of movable burners, operative on all of the reheatcrs. The supplementary coupled control means may then take the form of water injectors operating on the high pressure steam before its application to the reheater heat exchangers, the position of the movable burners and the volumes of injected water being subjected to control by a signal derived from the sum of the two reheat temperatures. Conversely the signal employed for control of the quantity of injected water may be made dependent on the difference of the two reheat temperatures.

The invention will now be described in further detail by reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a steam generator according to the invention including two tube systems, movable burners disposed in a common firing space, and water injector means operating on the steam at high pressure before its entry into heat exchangers traversed by partly expanded steam;

FIG. 2 is a diagram similar to that of FIG. 1 but showing a modification of the invention relating particularly to the control apparatus;

FIG. 3 is a diagram of a steam generator according to the invention having two tube systems and movable burners in a common firing space, and bypass means for the steam to be reheated about a part of the reheater heating surfaces;

FIG. 4 is a schematic diagram of another steam generator according to the invention having two tube systems and movable burners in a common firing space, and having means for bypassing part of the high pressure steam about the heat exchangers in which the partly expanded steam is to be reheated by that high pressure steam; and

FIG. 5 is a diagram similar to that of FIG. 1 but showing a further modification of the invention including recirculation of the products of combustion in the boiler and injection of water into the high pressure steam before its introduction into the heat exchangers.

In the steam generating system of FIG. 1, steam or other vaporized working substance emerging from feed water preheating and vaporizing devices not shown enters one of two parallel-connected tube systems at 11 and passes through an initial superheater 12. In this tube system it then passes through a line 13 into a heat exchanger 14, preferably disposed within the radiation zone of the boiler fire. In the embodiment shown the steam emerging at high pressure from line 13 passes through the inner tube 15 of the exchanger 14 whereas partly expanded steam is passed through the outer tube 16 which surrounds the inner tube 15. The high pressure steam finally passes through a third superheater surface 17 and through the line 18 into the high pressure end of a turbine, not shown. Partly expanded steam extracted from the turbine enters the exchanger l14 at a line 19, passing through the outer tube 16 thereof. In the heat exchanger 14 this partly expanded steam will be either heated or cooled, according to the relation of its temperature to that of the high pressure steam in the inner tube 15. If desired, superheater elements, heated by the products of combustion, may be inserted into the flow path of the partly expanded steam.

The partly expanded steam after passage through the exchanger 14 passes through a line 20 into the portion 21 of the r-eheater, for re-superheating of the partly expanded steam.

The steam so reheated passes through the line 22 into the low pressure portion of the turbine and is thence delivered to a condenser (not shown) in known fashion.

The elements 11 to 22 constitute part of a first tube system. A second tube system includes corresponding elements identified at corresponding reference characters 11' to 22.

For control of the temperature of the high pressure steam delivered at line 18' of the second tube system, there is provided a water injector line 23' which includes a rate of flow control device 24' therein. Line 23' opens into the final superheater 17'. The control device 24' is made responsive to temperature measurements at the inlet and outlet ends of the superheater 17', such measurements being made by temperature measuring devices 26 and 25'.

A similar temperature regulating system including elements 23, 24, 26 and 25 is associated with the superheater 17 of the first tube system.

The steam generator is fired by means of movable burners 31 and 31' which are disposed in a single firing space including both of the tube systems which have been described. The two burners are mechanically linked together by means of linkages diagrammatically indicated at 32, 33 and 34. A tubular element 35 is affixed to the support bar 33, and the element 35 is supported at bearings 36 for translatory motion as indicated by the arrows 37. Motion of the bar 33 downward in the figure, in the direction identified by the minus sign associated with arrows 37, shifts the burners in the direction identified by the minus signs associated with arrows 38, and vice versa.

The movable burners constitute a first coupled means for adjustable control of the heat absorption in the two tube systems, i.e. changes produced by this means being in the same sense at both tube systems. The setting of these means is governed by a control signal which is a function of the reheat temperatures in the two tube sys tems. To this end temperature measuring elements are provided at 41 and 42 in the lines 22 and 22'. These temperature measuring devices generate temperaturerespresentative signals in form of pressures in the hydraulic lines 43 and 44 by conventional apparatus, the details of which are not shown. Upon departure of the temperatures and t measured at elements 41 and 42, from reference levels which are controllably set by apparatus not shown, variations occur in the pressure signals in the lines 43 and 44 which are added in the hydraulic piston system indicated at the dash line box 45. This system includes pistons 47 and 48 coupled together by means of a rod 46. The pistons 47 and 48 are disposed in separate cylinders 49 and 50 to which communicate respectively the lines 43 and 44. The stress on rod 46 is hence a measure of the sum of the pressures in lines 43 and 44. The rod 46 is coupled to a control piston 51 which in known fashion generates in a hydraulic line 52 a pressure signal representative of the sum of the two temperatures. The pressure in line 52 operates on the body 53 of a valve 53' forming part of a servo motor which includes a fixed cylinder 54 and a piston 55 therein, controlling the position of a rod 56. A plate 57 is pinned to the rod 56 and a spring 58 is engaged between the plate 57 and the bar 33.

If for example the sum of the temperatures t and t rises above the reference level therefor, the rod 46 moves in the downward or negative direction indicated by the negative sign associated with arrows 59. The pressure signal thus generated in line 52 pushes the valve body 53 downward, permitting hydraulic fluid to flow into the valve 53 under pressure at a line 60 from a reservoir not shown. This fluid passes through valve 53 to the upper side of piston 55, lowering the latter, causing a lowering of the bar 33 in the negative sense of arrows 37 and a shift of the movable burners in the negative sense of their arrows 38. Heat transfer in the two tube systems is thereby changed, in the same sense for both, diminishing the heat applied to the reheaters which comprise exchangers 14 and 14 and superheaters 21 and 21'. The sum of the two temperatures and t accordingly constitutes a control signal for the system shown, dependent on the average value of the two reheat temperatures.

Shift in the position of the burners is limited by stops 61 on the motion of bar 33.

In order to extend the range of control over the reheat temperatures, the generator of FIG. 1 includes supplementary coupled control means on the heat transfer to the two tube systems. These take the form of water injectors, for injecting water from the line 71 into the lines 13 and 13. For control of the volume of water injected, rate of flow control devices 72 and 72' are provided whose rates of flow are adjusted together in the same sense by motion of the rod 73. This rod controls the two valves 72 and 72' by means of a lever 74, to whose midpoint it is pivotally coupled. Rod 73 is coupled to rod 56 via a link generally indicated at 75 comprising a cage or cylinder 75 attached to rod 56 and a piston 75" fitting loosely in the cage, the piston being attached to rod 73. Link 75 thus provides a specified amount of play between rods 56 and 73, parallel to their own length.

So long as no positive coupling exists between rods 56 and 73, by virtue of travel of the piston 75" in cage 75', rod 73 will be held by spring 76 in the position defined by abutment of plate 77' (pinned to red 73) on stop 77. Spring 76 is engaged between the plate 77' and an abutment 77".

If however rod 56 is drawn downward sufiiciently by servo piston 55, the bar 33 will rest on stops 61 and the piston within cage 75' will reach the end of its travel, providing positive mechanical coupling between rods 56 and 73. Rod 73 will then with further downward travel of rod 56 also be drawn downward, in the direction of the negative sign associated with arrows 78. In this way the openings of valves 72 and 72' are both enlarged. The result is an increase in the rate of water injection into both tube systems and a consequent cooling of the high pressure steam before its entrance into the heat exchangers 14 and 14. Accordingly the temperatures of the partly expanded steam emerging from the heat exchangers are likewise reduced, and the reheat temperatures of the reheated steam decline at lines 22 and 22'.

It will be seen that the supplementary coupled control means on the heat transfer in the two tube systems constituted by the water injector apparatus shown will come into operation only when the first coupled control means comprising the movable burners have reached the limit of their travel. In this way the quantities of injected water are under normal circumstances held to low values, which is thermodynamically advantageous.

According to the invention there is combined with the control signal which operates on the water injectors a supplementary control signal which is a function of the temperature of the reheated steam. To this end the apparatus enclosed within the dash line box 81 develops a signal corresponding to the difierence between the two reheated steam temperatures t; and The pressure in the line 44 is applied to a piston 82 in a fixed cylinder 83 and the pressure in line 43 is applied to a similar piston 84 in a fixed cylinder 85. The motions of the pistons which occur upon changes in temperatures 1 and t, are subtracted from each other by means of the lever 86 to which the pistons are coupled, and the resultant motion of the rod 87 coupled to piston 84 controls the position of a control piston 88. In this way there is developed in the line 89 a pressure dependent on the difference between the temperatures 2 and t and this pressure is applied to a piston 91 in a fixed cylinder 90. A spring 92 engaged between this piston and cylinder 90 stresses the piston toward a limit position, and rod 93 pinned to piston 91 is coupled via lever 94 and link 95 to arm 96 of lever 74.

If the reheated steam pressure is above the reference level established therefor, the pressure in the line 89 declines and piston 91 moves downwards. This rotates lever 74 in such a sense as to increase the rate of water injection into line 13' and to reduce the rate of water injection into line 13. This produces a drop in temperature 1 Temperature t will stay at the reference level established therefor notwithstanding the reduced injection of water into line 13, since with the increase in temperature t supposed, the sum of the two temperatures will have risen above the reference level for the sum, with consequent shift in the position of the burners in the sense identified by the minus signs associated with arrows 38. This compensates for the reduced water injection into line 13. It is worth repeating that the adjustment of both of injector valves 72 and 72' in the same sense by rod 73, i.e. to open both or to close both, increases the control range for the temperatures and t and that on the other hand the rotation of lever 74 superposed by linkage 94-95 on the motion of that lever produced by rod 73 provides a diiferential or opposite-sense control of the reheat temperatures.

In the steam generator of FIG. 2 there are provided two tube systems as in that of FIG. 1, like reference characters indicating like elements of structure. The first coupled means for control of the reheater temperatures comprise again movable burners 31 and 31' disposed with the tube systems in a common firing space. The position of these burners, which move together in the same sense with respect to increase or decrease of heat applied thereby to their tube systems, is in this embodiment controlled exclusively by the temperature measured at a device 101 in line 22. The temperature measuring device 101 consequently generates a pressure signal, in a known manner, in the hydraulic line 102. This signal, representative of the temperature is communicared through a bellows 103, into motion of a rod 104 and thence to lever 105 which adjusts a servo control valve 106. This governs motion of a servo piston 107 and of the bars 108 and 109 which specify the position of the burners. The elements are so proportioned that increase in the temperature t above a reference level, which can be adjustably set by means not shown, shifts the burners in the direction indicated by the minus sign associated with arrows 110. This reduces the application of heat to the reheating coils 21 and 21'. In consequence, temperatures is returned to its reference value.

For extension of the control range on the reheat temperatures supplementary means are provided, as in the embodiment of FIG. 1, for control of the heat transfer to the two tube systems. These supplementary means may take the form of a water injector system for adjustment in the same sense of the rate at which water is injected from the conduit 111 into the high pressure steam lines 13 and 13. For this adjustment valves 112 and 112' are cont-rolled by a linkage comprising a rod pinned to bar 108 and levers 113 and 114 coupled to bar 115. The opening of both valves is thus changed in the same direction simultaneously with change in the position of the burners, it being assumed for the moment that the right hand end of lever 114 constitutes a fixed fulcrum. A value for the temperature r above the correct or reference value therefor thus produces not only a shift in the position of the burners for reduced heat application as already described but also an opening of both valves 112 and 112. The provision of valves 112 and 112' thus connected to the mechanism for change in the burner position thus extends the control range on the temperatures and whereby desired reheat temperatures can be maintained over a wider range of loads on the system, corresponding to a wider range of steam flow rates.

Upon the control just described there is superposed an additional control signal derived from the temperature I, and which operates exclusively on the tube system 13', of whose temperature 2' is a measur The temperature t develops by known means a pressure signal in the line 122, derived from a measurement device 121. This pressure signal is transmitted through the bellows 123, control piston 124, hydraulic line 125 and position control piston 126' in a cylinder 126 to a rod 127 whose motion changes the fulcrum point for lever 114 with respect to its rotation by rod 115. The system is so established that with an increase in the temperature t the piston 126 eflects an opening in valve 112', and vice versa.

In the steam generator of FIG. 3 there are again provided two steam tube systems of which for simplicity there are shown only the initial superheaters 131 and 131, the final superheaters 132 and 132', and the reheaters 133 and 133. The first means for coupled control (i.e. in the same sense) of the rate of heat transfer to the two tube system again comprise movable burners 134 and 134'. Their position is controlled. by the level of the temperature t which is made effective through a hydraulic line 135 on servo control pistons 136 and 136 linked to the burners. The operation may be that described in conjunction with FIG. 2.

For increase in the control range With respect to the temperatures t and t there are provided additional coupled means for shifting in the same sense the rate of heat transfer to the two tube systems. These supplemenary means comprise rate of flow control valves 137 and 137' disposed in bypass lines 138 and 138' which are connected in parallel with the intermediate reheater superheater surfaces 133 and 133'. According as more or less of the working substances flows through the lines 138 and 138, the temperatures t and 2 are lowered or raised.

The two valves 137 and 137' are subjected to the same control signal, as are the positions of burners 134 and 134'. The line 135 is connected through an extension line 139 to servo control pistons 137a and 137b which are coupled to the valves 137 and 137' respectively. A temperature above the correct value effects an opening in both of the valves 137 and 137'.

On the control signal applied via line 139 to the control piston for valve 137 there is superposed l8. control signal which is a function of the temperature t and which serves for adjustment of that valve alone, with consequent efiect upon the temperature t A temperature t above the reference value effects opening of the valve 137 and vice versa, by operation on the servo control piston 137a through a hydraulic control signal line 140.

In the steam generator system of FIG. 4 as in that of FIG. 1 there are two separate steam tube systems, identified with the same reference characters as in FIG. 1 for corresponding elements or structure. The first coupled means for adjusting, in the same sense, the rate of heat transfer in the two tube systems comprise again movable burners 141 and 141'. The position of these burners may be controlled by mechanism similar to that in FIG. 3 including a temperature-representative signal generating device 1 connected through a signal channel 2 with servo control pistons 3 and 3' coupled to burners 141 and 141'. In contrast to the embodiment of FIG. 3, however, the supplementary means for coupled control of the rate of heat transfer to the two tube systems comprises rate of flow control valves 142 and 142, disposed in bypass lines 143 and 143' which are connected around the inner coils 15 and 15 of heat exchangers 14 and 14' respectively. According as a greater or lower quantity of steam flows through the bypass lines, the reheat temperatures t and are lowered or raised. Adjustment of the two valves 142 and 142' together in the same sense extends the control range of the reheat temperatures as does coupled adjustment of the valves 137 and 137' in the embodiment of FIG. 3. The valves 142 and 142' are controlled for motion together in the same sense by servo control pistons 4 and 4, which are connected to the signal channel 2. In the embodiment of FIG. 4 there is added by means of a temperature-representative signal generating device operating via signal channel 6 on servo control piston 4 of left-hand valve 142 alone a supplementary control signal dependent on the temperature i In the steam generator of FIG. 5 the working substance enters, from a feed water p-reheater not shown, into the vaporizing coil 151 of one tube system and passes therefrom into an initial superheater 152. It then passes through a conduit 154 into the coil 156 of a heat exchanger 155, disposed outside the wall of the firing space, shown fragmentarily at 149. In the exchanger 155 the newly vaporized steam entering through line 154 serves either to heat or to cool the partially expanded steam fed from the turbine to the heat exchanger at line 157. The high pressure steam then passes from coil 156 through a second superheater 158 and through a conduit 159 to the high pressure end of the turbine, not shown. Partially expanded steam extracted from the turbine passes, as already stated, through the line 157 into that portion of heat exchanger 155 outside the coil 156 and from thence to the reheater surface 160, indicated diagrammatically as a coil. The steam so reheated passes through the line 161 into the low pressure end of the turbine and is withdrawn therefrom to a condenser.

In FIG. 5 the elements identified with reference characters 151 to 161 form part of the first tube system. A second corresponding tube system includes elements identified at reference characters 151 to 161'. Both tube systems are heated in a common firing space by means of a burner 162.

As a first coupled means for changing in the same sense the rate of heat transfer in both tube systems there is provided a flue gas return arrangement comprising a conduit 162' in which there is provided a blower 163. A portion of the stack gases is returned by these means from the upper end of the boiler to a point closer to the fire. The quantity of the products of combustion so recirculated can be adjusted by means of a damper valve 164, coupled to a rod 165. Rod 165 is in turn coupled to a lever 166 to whose midpoint is pivotally attached a bar 167. So long as the lever 166 stands clear of the stop 168, springs 169 and 170 maintain the lever 166 in a specified angular position, e.g. perpendicular, to the bar 167. The bar 167 may be controlled in position by a servo motor 54 as illustrated in FIG. 1 with respect to the bar 33 of that figure. Adjustment in the position of bar 167 is thus so made that when the sum of reheat temperatures r and t in lines 161 and 161 of FIG. 5 lies above the sum of their reference values, the quantity of recirculated flue gases is reduced by lowering of bar 167, which produces a partial closing of damper 164, and vice versa-so long as bar 166 clears stop 168.

The supplementary coupled means for controlling in the same sense the rate of heat transfer in the two tube systems are again, as in the embodiment of FIG. 1, provided in the form of Water injectors by means of which water provided at a conduit 172 can be injected into the high pressure steam lines 154 and 154 through additional conduits 173 and 173' respectively. The openings of valves 174 and 174 in these last-named lines are changed together in the same sense by motion of the bar 175, which is coupled through a limited lost motion linkage 176 like the linkage 75 of FIG. 1 with the left end of the lever 166. As in the embodiment of FIG. 1 this arrangement results in adjustment of the supplementary control means at valves 174 and 174' only when the control range of the first means comprising valve 164 has been exhausted by engagement of bar 164 against stop 168.

For a differential or individual adjustment of the reheat temperatures the lever 177 may be rotated about its point of connection to lever by means similar to those described in connection With FIG. 1 as operative on the lever 74 of that figure.

It will be seen from. the foregoing that the invention provides, in steam generators having plural heaters and tube systems, improved means for control of reheat temperature by control of the heat source or sources which operate on the reheaters, either directly or via freshly vaporized steam brought into heat exchange relation With the steam to be reheated, by control of the temperature of that fresh steam through water injection or like means, by control through bypass means of the relative quantities of fresh and reheat steam so brought into heat exchange relationship, by control of recirculation of stack gases flowing over fresh steam or reheat steam coils or both, by the provision of bypass means about such reheat steam coils, and by combinations of these arrangements, and by application of these control means individually or difierentially to the separate reheaters, as cumulatively thereto.

The invention is not limited to the embodiments which have been described hereinabovc. Of course, the heating exchange surfaces schematically shown in the drawings may comprise plural tubes or groups of tubes. in all embodiments moreover various control means are provided in the high pressure steam tube systems, operating with respect to rate of steam flow, pressure and temperature. The invention is applicable to both forced circulation and natural circulation boilers, but is employed with particular advantage in steam generating plants operating at supercritical conditions.

I claim:

1. A steam generator comprising firing means, at least two parallel-connected tube systems each including a heheater, said generator further comprising first and second means for varying in the same sense the amount of heat transferred to partially expanded steam in each of said reheaters, means responsive to the temperature of the steam reheated in at least one of said reheaters for operating said first and second means to increase said amounts upon decline of said temperature and vice versa, and supplementary means responsive to the temperature of the steam reheated in one of said reheaters for additionally varying the amount of heat transferred to partially expanded steam passing through said one reheater.

2. A steam generator comprising firing means, at least two parallel-connected tube systems each including a reheater, said generator further comprising first and second means for varying in the same sense the amount of heat transferred partially expanded steam in each of said reheaters, means responsive to change in the sum of the temperatures of the steam reheated in said two reheaters for operating said first and second means to increase said amounts upon decline of said sum and vice versa, and supplementary means responsive to change in the difference between the temperatures of the steam reheated in said two reheaters for additionally varying in opposite snses the amounts of heat transferred at said two reheaters to partially expanded steam passing therethrough, said last-named means varying, as to that one of said tube systems in whose preheater the change of temperature predominates in a change of said difference, the heat transferred to such tube system inversely with the sign of the change in reheated steam temperature in such tube system.

3. A steam generator comprising firing means, a plurality of parallel-connected steam tube systems, a reheater associated with each of said tube systems, first and second reversibly movable means to change in the same sense the amount of heat transferred to partially expanded steam in each of said reheaters, a lost motion linkage means coupling said first and second means together, and means responsive to variation in the temperature of steam reheated in at least one of said reheaters to move one of said first and second means in the sense required to vary the rate of heat transfer in each of said reheaters as an inverse function of said steam temperature.

4. A steam generator comprising two tube systems connected in parallel for the flow of a Working substance between a condenser and a turbine, two reheaters connected in parallel for the flow of a working substance between a higher and a lower pressure stage of the turbine, a separate source of heat associated with each of said tube systems, separate first means to vary the amount of heat absorbed from said sources by partially expanded steam in each of said reheaters, separate second means to vary the amount of heat absorbed from said sources by partially expanded steam in each of said reheaters, first and second movable coupling means respectively coupling said separate first varying means together and coupling said separate second varying means together such that motion of either of said coupling means shifts both of the varying means coupled thereto in the same sense, a linkage including a lost motion for coupling said first and second movable coupling means to each other, motor means responsive to variations in the temperature of the partially expanded steam reheated in at least one of said reheaters, said motor means being coupled to one of said first and second movable coupling means to move said one of said coupling means in the sense required to vary the absorption of heat by each of said preheaters as an inverse function of said temperature and supplementary motor means responsive to variations in the temperature of the partially expanded steam reheated in one of said reheaters coupled to one of said first and second separate means for additionally varying the absorption of heat by partially expanded steam in said one reheater as an inverse function of said last-named temperature.

5. A steam generator comprising firing means, a plurality of tube systems connected in parallel between a condenser and a high pressure stage of a turbine, a reheater associated with each of said tube systems, said reheaters being connected between two different pressure stages of said turbine, each of said reheaters comprising means to exchange heat between fresh steam in one of said systems and partially expanded steam and a reheater coil adapted to absorb heat from said firing means, said generator further comprising first reversibly movable means to change in the same sense the rate of heat exchange between said firing means and each of said tube systems, second reversibly movable means to change in the same sense the rate of heat exchange between fresh steam and partially expanded steam in each of said reheaters, a lostmotion linkage means coupling said first and second reversibly movable means together, and supplementary means responsive to variation in the temperature of steam reheated in at least one of said reheaters to additionally move one of said first and second reversibly movable means in the sense required to vary the rate of heat exchange controlled by said one of said reversibly movable means as an inverse function of said last-named temperature.

6. A steam generator comprising a plurality of steam tube systems each including a reheater, each of said reheaters including means to exchange heat with its associated tube system, said generator further including separate movable firing means for each of said systems, a separate flow regulating means for admission of water to each of said systems in advance of its heat exchange means, first movable means coupling said firing means together for reversible motion in the same sense with respect to their tube systems, second movable means coupling said flow regulating means together for reversible adjustment in the same sense, means linking said movable coupling means together, first motor means responsive to variations in the steam temperature in at least one of said reheaters to shift said linking means in the sense required to vary the exposure of said tube systems to said firing means as an inverse function and the opening of said flow regulating means as a direct function of said steam temperature and second motor means responsive to variations in the steam temperature in one of said reheaters to shift one of the firing and flow regulating means of the tube system including said one reheater in the sense required to vary the exposure of said last-named tube system to said last-named firing means as an inverse function and the opening of said last-named flow regulating means as a direct function of said last-named temperature.

7. A steam generator according to claim 1 wherein each of said reheaters includes a bypass line and wherein one of said first and second means comprises a flow control means in each of said by-pass lines.

8. A steam generator according to claim 7 wherein each of said reheaters includes a fresh steam line and a line for partially expanded steam, said lines being in heat exchange relation, and wherein said by-pass lines each by-pass one of the fresh steam and partially expanded steam lines of a separate one of said reheaters.

9. A steam generator according to claim 1 wherein each of said reheaters comprises a fresh steam line and a partially expanded steam line in heat exchange relation, wherein one of said first and second means comprises adjustable means to recirculate over said tube systems the References Cited in the file of this patent UNITED STATES PATENTS 1,930,456 Rumble Oct. 10, 1933 2,752,899 Kasak July 3, 1956 2,918,909 Nickel Dec. 29, 1959 FOREIGN PATENTS 758,833 Great Britain Oct. 10, 1956 

1. A STEAM GENERATOR COMPRISING FIRING MEANS, AT LEAST TWO PARALLEL-CONNECTED TUBE SYSTEMS EACH INCLUDING A REHEATER, SAID GENERATOR FURTHER COMPRISING FIRST AND SECOND MEANS FOR VARYING IN THE SAME SENSE THE AMOUNT OF HEAT TRANSFERRED TO PARTIALLY EXPANDED STEAM IN EACH OF SAID REHEATERS, MEANS RESPONSIVE TO THE TEMPERATURE OF THE STEAM REHEATED IN AT LEAST ONE OF SAID REHEATERS FOR OPERATING SAID FIRST AND SECOND MEANS TO INCREASE SAID AMOUNTS 